Moving coil type loudspeakers



J. G. CARTER MOVING COIL TYPE LOUDSPEAKERS Filed Feb. 28, 1955 Dec. 31, 1957 United States Patent MOVING COIL TYPE LOUDSPEAKERS John George Carter, Ashford, England, assignor to Goodmans Industries Limited, Wembley, England, a British company Application February 28, 1955, Serial No. 491,145

Claims priority, application Great Britain March 5, 1954 3 Claims. (Cl. 179115.5)

This invention relates to moving coil type loudspeakers and more particularly to coaxial loudspeakers of the kind embodying two separate moving coil systems, one for high frequency and the other for low frequency reproduction, filters being used to limit the frequency range of each system to the desired frequency band.

It is the object of the present invention to provide an improved magnet system for loudspeakers of the above kind which simplifies manufacture and assembly and which is more economical in its use of magnetic material than known instrtunents of this kind so reducing production costs.

According to the invention in a loudspeaker of the kind mentioned the magnet assembly comprises a centrepole member co-operating with a front plate or the equivalent to form a first air gap, said pole member being formed or provided at its front end with an auxiliary pole element of smaller diameter which co-operates with a supplementary pole plate to define a second air gap concentric with the first and located in the same magnetic circuit.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein Fig. 1 is an axial cross section through the improved loudspeaker unit and Fig. 2 is a similar view to a larger scale of the auxiliary magnet system.

The unit shown comprises a basic magnet structure of known type consisting of a pot-like casing 1 formed of for example mild steel, a centre pole member 2 and a front pole plate 3 secured to the rim portion of the easing and co-operating with the member 2 to form a first annular air gap 4, the centre pole member in this instance consisting of a magnet 2a and a pole tip 2b which are clamped together and to the casing by a central bolt 5. For the purpose of the present invention an annular groove or recess 6 is cut in the peripheral portion of the front end of the pole tip 2b in such a manner as to remove a portion of the peripheral face and a portion of the front end face of said pole tip so leaving a short auxiliary centre pole element upstanding from the body portion of the pole tip and of smaller diameter than said body portion. Mounted within the annular groove 6 upon a non-ferrous locating ring 7 seated in the groove is a supplementary ring-shaped pole plate 8 formed of mild steel or of a high permeability alloy, said plate being of less thickness than the front pole plate 3 and being so disposed that its front face lies approximately flush with the front faces of the pole plate 3 and of the auxiliary centre pole element 20. The outer or peripheral edge of the plate 8 is aligned with the peripheral face of the pole tip 2b but is slightly spaced therefrom whilst its inner edge co-operates with the auxiliary centre pole element 2c to define a second and smaller annular air gap 9 concentric with the main or outer air gap 4.

Thus the supplementary pole plate 8 in conjunction with said auxiliary centre pole element 20 forms an auxiliary magnet system, a portion of the magnetic flux in the main or outer air gap being conducted along the plate 8 to the inner or auxiliary air gap 9, and the proportion of flux thus deflected can be predetermined by the axial location of the supplementary pole plate relative to the main pole tip 2b.

Located within the outer air gap 4 is the driving coil 10 of a low frequency, cone type diaphragm 11 which may be supported in the usual manner within a chassis 12 to the rear side of which is attached the magnet unit. Disposed within the inner air gap 9 is the driving coil 13 of a high frequency diaphragm 14 which preferably and as shown consists of a dome-like structure enclosing the tip of the auxiliary centre pole element 20 and itself enclosed by a perforated guard or cover 15. The high frequency system is attached to the front end of the centre pole member, the preferred arrangement being as follows:

The high frequency diaphragm is supported by a centering device 16 which is clamped between a pair of Washers 17 seated in a recess formed in the upper side of the supplementary pole plate 8, said Washers, the supplementary pole plate and the locating ring 7 being secured in position by a number of non-ferrous bolts, one of which is shown at 18, extending through the assembly and into the pole tip 2b. Conveniently, said bolts also serve to clamp the cover 15 in position, a suitable spacer ring 19 being interposed between said cover and the washer 17 as shown. The high frequency unit may include a small acoustic horn 20 which is then also secured in position by the clamping bolts, whilst if desired the centering device 21 of the low frequency diaphragm may be clamped between the bolting fianges of the cover 15 and the born 20.

It will be understood that the embodiment illustrated is given by way of example only and is capable of several modifications. Thus in practice the locating ring 7 may be relieved to meet acoustic requirements or it could be replaced by non-ferrous distance pieces. Again, the groove or recess 6 could be of different shape or the auxiliary centre pole element 20 could be made separately and be bolted in position in which case it could be made of high permeability alloy. It is also to be understood that the invention is equally applicable to a ring-type permanent magnet assembly or to a mains energised magnet assembly.

With the invention, it has been found that an almost negligible amount of extra magnetic material is required in the centre pole to maintain the required flux density in each of the two air gaps and thus the invention provides a very simple, effective and economical magnet assembly for a coaxial loudspeaker.

I claim:

1. A loudspeaker comprising a main center pole member, a main annular pole piece cooperative with said center pole member and defining a first air gap, an auxiliary center pole member of smaller diameter than the main pole member and situated at the end of the latter, an auxiliary annular pole plate cooperative with said auxiliary pole member and defining a second air gap coaxial with the first air gap, a non-magnetic spacing member surrounding the auxiliary pole member and adapted to hold said auxiliary pole plate away from the main pole member, a first moving coil system located in the first air gap, and a second moving coil system located in the second air gap.

2. A loudspeaker as defined in claim 1, wherein said main center pole member has an annular groove in the periphery of the end thereof which forms said auxiliary center pole member, and said non-magnetic spacing memplate and connecting said low-frequency diaphragm to bet is located in said groove. said first moving coil system.

3. A loudspeaker as defined in claim 1, including a References Cited in the file of this Patent high-frequency diaphragm directly connected to said second moving coil system, a low-frequency diaphragm, and 5 UNITED STATES PATENTS means passing around the outside of said auxiliary pole 2,699,472 Olson et a1. Jan. 11, 1955 

